The telescope is not at fault: it has come from a 20 °C room into an 8 °C night, and while its mirror is warmer than the air it heats the air in front of it. You have to wait for it to reach the outside temperature. This page says why, how long, and how to shorten the wait.
The short answer
- The symptom: an image that shimmers, a planet whose details come and go, a star that never becomes a sharp point however you focus.
- The cause: a mirror or lens warmer than the air. Air warmed against it rises and churns in the tube and just in front of the optics.
- The cure: time. The image settles once the instrument is within about a degree of the air.
- It is not a fault: every telescope does it. A large mirror does it for longer than a small lens.
How long, by instrument
- Refractor: the quickest. A lens is thin and the tube is closed: half an hour is often enough, and tube currents are rare.
- Newtonian and Dobsonian: the mirror is a thick glass disc at the bottom of an open tube. Allow an hour for a 130 or 150, longer for a 200 or 250, and longer still when the temperature gap is large.
- Maksutov and Schmidt-Cassegrain: closed tube, thick front corrector, little airflow. The slowest of all, and it is their best-known weakness.
There is no exact rule in minutes per millimetre: it depends on the gap between the room and the night, the thickness of the mirror, and the wind. The times above are observers' orders of magnitude, not a calculation. The only judge is the image: when a star just out of focus shows even rings instead of a smoking blob, you are ready.
How to speed it up
- Take the instrument out an hour before observing, cap off if the air is dry, protected from dew if not. Set up while it is still light.
- Store it somewhere cool: a garage, a shed, a dry cellar. It starts from closer. This is the most effective step, and it is free.
- A small fan behind the mirror, on Dobsonians that have a mount for one, cools it markedly faster.
- Observe away from heat: not over a roof, a concrete patio that has been in the sun all day, or in front of an open window into a heated room.
Meanwhile the time is not lost: look at the Moon, an open cluster, anything at low power. The shimmer matters little when you are not magnifying much, and your eyes need twenty minutes to adapt to the dark anyway.
Or else it is the atmosphere
If the image still moves after two hours outside, the instrument is no longer to blame: it is turbulence in the atmosphere, which varies from night to night. Stars twinkle hard to the naked eye on those evenings. There is nothing to do except magnify less and come back another night.
Questions people ask
Why is my telescope's image blurry?On a first night, most often because the instrument is warmer than the air: the mirror heats the air in front of it and the image shimmers.
Wait half an hour to an hour outside. If it persists, it is atmospheric turbulence, or the focus needs redoing.
How long should a telescope cool down?About half an hour for a small refractor, an hour for a 130 or 150 mm Newtonian, longer for a large Dobsonian or a Maksutov, especially in winter.
These are orders of magnitude; the image tells you when it is ready.
Should I store my telescope outside?
Not outside, but somewhere cool: a dry garage or shed saves it starting from 20 °C every evening, and it is the simplest way to shorten the wait.
What does a fan on a telescope do?It cools the mirror faster and breaks up the layer of warm air clinging to its surface.
On a Newtonian or Dobsonian of 200 mm or more, it is the clearest gain.
How do I know the telescope has cooled?
Look at a bright star just out of focus: even, steady rings mean it is ready; a blob that smokes or ripples on one side means wait longer.
Method and changes
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